Role of humic acids in the TiO2-photocatalyzed degradation of tetrachloroethene in water

The effect of humic acids on the TiO2-mediated photocatalytic degradation o f tetrachloroethene (PCE) was kinetically investigated at different pH and initial substrate concentrations. The pro cess occurs through two parallel paths: a major oxidative route leading to mineralization and a reductive ro ute leading to the formation of dichloroacetic acid (DCAA), also undergoing photodegradation. The rate of PCE decomposition was found to decrease in t he presence of humic acids adsorbed on the semiconductor surface, while the concentration of the intermediate dichloroacetic acid increased. This is a consequence of the scavenging action bf humic acids toward photoproduced s urface oxidant species, which makes conduction band electrons more easily a vailable for interface reactions. Kinetic studies on the effect of humic ac ids in the TiO2-mediated photodegradation of dichloroacetic acid showed tha t the progressively greater accumulation of this highly toxic intermediate, observed with increasing humic acids content, was a consequence of both an increase in the rate of its production from PCE and a decrease in the rate of its oxidative photodegradation. (C) 1999 Elsevier Science Ltd. All righ ts reserved.